1. Field of the Invention
The invention generally relates to battery chargers; and in particular, the present invention relates to a method and an apparatus for monitoring the self-discharge of a secondary battery.
2. Background of the Invention
A conventional battery charger for lithium-ion (Li+) batteries charges a battery until either a predetermined period of time has elapsed or the charge current drops below a predetermined threshold. Once the charge cycle is terminated, the charge current is interrupted so that no current flows into the battery. The interruption of the charge current is necessary because an indefinite flow of charge current into a Li+ battery that has already reached its prescribed maximum allowable terminal voltage will degrade the battery's life cycle.
After the charge cycle is terminated and the charge current is interrupted, the charger enters a float period, also called a monitor period, where the battery stays connected to the charger but is allowed to float. The float period lasts until the battery is removed from the charger for use in a desired application. Thus, the float period can last indefinitely.
If the battery is not removed from the charger for use immediately after it is fully charged, the energy stored in the battery will gradually leak away. Two mechanisms account for the leakage of energy during the float period. First, lithium ion batteries are susceptible to self-discharge. Therefore, a Li+ battery left sitting on a shelf will eventually lose all of its stored energy. The self-discharge mechanism is a function of the battery chemistry and physical construction, and cannot be prevented. Because it is desirable to maintain a battery at its full capacity while the battery is left in the charger, a conventional battery charger monitors the capacity of the battery during the float period, and takes measures to recharge, or refresh, the battery back up to its full capacity whenever energy is lost through self-discharge. However, the monitor circuit used in most conventional chargers significantly contributes to the second leakage mechanism of Li+ batteries during the float period.
A charger controller in a conventional battery charger uses either an external or an internal shunt resistor network to monitor the terminal voltage across a battery. FIG. 1 illustrates a conventional battery charger employing an external shunt resistor network connected to a charger controller 10. The shunt resistor network includes a first resistor 12 and a second resistor 14 which are connected in series. One side of first resistor 12 is connected to a Battery High Side terminal 22. One side of second resistor 14 is connected to a Battery Low Side terminal 24. The node between first resistor 12 and second resistor 14 is connected to a BAT input of charger controller 10. The resistor network senses the battery voltage across High Side terminal 22 and Low Side terminal 24 and provides an attenuated version of the battery voltage between the BAT and SNS inputs of charger controller 10. A third resistor 16 monitors the current flow into the battery. Use of a shunt resistor network to monitor the battery's terminal voltage has several disadvantages.
First, in the shunt resistor network of FIG. 1, measurement accuracy of the attenuated voltage is set by the resistor matching accuracy. When external resistors are used, non-standard resistor values with high accuracy are required to provide the proper resistor ratio set points. The resistor requirements can significantly increase implementation cost. Second and more importantly, the shunt resistor network provides a current path for discharging the battery. If the resistor values used in the divider network are too large, measurement accuracy is compromised by noise susceptibility. However, if the resistor values used in the divider network are too small, the battery will be excessively loaded. The battery can be drained of its stored energy by the monitoring process. Thus, the shunt resistor network is undesirable for use in a battery charger because the shunt resistor network depletes the battery's energy while monitoring its terminal voltage.
Therefore, it is desirable to provide a means to monitor the battery voltage in a charger which does not contribute significantly to the discharge of the battery. A desirable charger controller should minimize the amount of energy loss attributable to monitoring of the battery's terminal voltage and be capable of refreshing a battery for energy lost due to self-discharge.
Another disadvantage of the shunt resistor network is that because the battery is discharged by the monitoring process, the battery is subjected to multiple discharge and recharge cycles in order to maintain a full capacity. For most type of rechargeable batteries, repeated discharge and recharge cycles reduce the useful life of the batteries. Therefore, it is desirable to minimize the number of times a battery has to be recharged back up to its full capacity during the float period.